feat(coverart): #388 remove global cover-art backfill cap

Operator feedback (2026-05-09): the 500-album cap meant cover art rolled
in over many nightly runs even when local sources (sidecar/embedded)
could finish in minutes. Remove the global cap; rely on the existing
per-provider HTTP throttle (coverart httpClient MinInterval) so local
art is disk-speed and remote providers stay TOS-friendly.

- enricher.go / artist_enricher.go: EnrichBatch, EnrichArtistBatch,
  EnrichRetryMissing now treat limit<0 as unbounded (0 still = stage
  disabled). The cap was a SQL LIMIT; unbounded uses max int32.
- main.go: RunScanConfig EnrichCap/ArtistEnrichCap = -1 (unbounded).
- Drop LibraryConfig.CoverArtBackfillCap + the
  MINSTREL_LIBRARY_COVERART_BACKFILL_CAP env var.
- Drop the now-dead coverBackfillCap param threaded through
  server.New + api.Mount + the handlers struct.
- Admin bulk refetch (/api/admin/covers/refetch-missing) now drains
  unbounded; response {queued:int} → {started:bool} (the count is
  unknowable synchronously for a fire-and-forget drain). Web copy +
  client type + Go/web tests updated to match.

No doc refs existed (config.example.yaml / docker-compose / README
never documented the env var).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -56,11 +56,10 @@ type LogConfig struct {
 }
 
 type LibraryConfig struct {
-	ScanPaths           []string `yaml:"scan_paths"`
-	ScanOnStartup       bool     `yaml:"scan_on_startup"`
-	CoverArtFromMBCAA   bool     `yaml:"coverart_from_mbcaa"`   // default true
-	CoverArtBackfillCap int      `yaml:"coverart_backfill_cap"` // default 500
-	ContactEmail        string   `yaml:"contact_email"`         // optional override for MBCAA UA
+	ScanPaths         []string `yaml:"scan_paths"`
+	ScanOnStartup     bool     `yaml:"scan_on_startup"`
+	CoverArtFromMBCAA bool     `yaml:"coverart_from_mbcaa"` // default true
+	ContactEmail      string   `yaml:"contact_email"`       // optional override for MBCAA UA
 }
 
 // SubsonicConfig controls wire-format concessions on the /rest/* surface.
@@ -120,8 +119,7 @@ func Default() Config {
 			RadioSizeMax:        200,
 		},
 		Library: LibraryConfig{
-			CoverArtFromMBCAA:   true,
-			CoverArtBackfillCap: 500,
+			CoverArtFromMBCAA: true,
 		},
 	}
 }
@@ -177,11 +175,6 @@ func applyEnv(cfg *Config) {
 			cfg.Library.CoverArtFromMBCAA = b
 		}
 	}
-	if v, ok := os.LookupEnv("MINSTREL_LIBRARY_COVERART_BACKFILL_CAP"); ok {
-		if n, err := strconv.Atoi(v); err == nil {
-			cfg.Library.CoverArtBackfillCap = n
-		}
-	}
 	if v, ok := os.LookupEnv("MINSTREL_CONTACT_EMAIL"); ok {
 		cfg.Library.ContactEmail = v
 	}